a) Field of the Invention
The present invention relates to a procedure for supplying combustion air during grate firings, in which primary combustion air is introduced through the fuel and secondary combustion air is introduced directly into the flow of exhaust gas and in which, in addition, some of the exhaust gas is tapped from the flow of exhaust gas and returned to the combustion process. The present invention also relates to a furnace for carrying out this procedure.
b) Background Art
In a mechanical furnace grate system, as a rule, the necessary combustion air that is added is divided into one part of primary combustion air, which is introduced into the fuel on the grate through the construction of the grate itself or through the rods that make up the grate, and secondary combustion air, which is introduced into the secondary combustion zone of the fire box above the furnace grate. In order to achieve complete combustion of both the solid and gaseous combustion residues, in all furnace grate systems, the combustion air is added in quantities that are above the stoicheomtrical quantity, i.e., more combustion air is added than would be required, in the ideal case, for the complete oxidation of the fuel. Each method of operation that is based on the above-stoicheometric quantities of necessity causes a reduction of the efficiency of a waste-heat boiler that is installed after the fire grate system, for the excess combustion air also has to be heated as ballast. In conventional combustion grate systems, this stoicheometric ratio lies in the range of 1.4 to 2.2.
Many furnace grate systems are divided into plurality of primary air zones in the direction of flow of the fuel. This division into different zones makes it possible to introduce primary combustion air as required, and in accordance with the particular conversion rate of the fuel. However, primary combustion air is frequently introduced into the rear area of the furnace grate, where, as a rule, there is only completely burned fuel, in order to cool the slag or to maintain the mechanical function of the grate surface. Within this area, there is no longer any conversion in the sense of a chemical reaction. Thus, in this area of the combustion chamber above the furnace grate there is a zone in which the gases are present, in slightly heated form, although their composition is very close to the composition of the primary combustion air that is introduced.
This portion of the primary combustion air, which no longer takes part in any chemical reaction, causes a considerable increase in the overall gas volume. A direct and disadvantageous consequence of this is that the apparatuses such as waste heat boilers or apparatuses used to scrub the exhaust gases, and which are incorporated after the combustion chamber have to be made correspondingly big, and are thus more costly. As has already been stated, the efficiency of such a furnace is also reduced by this excessive air ballast. Finally, this excessive proportion of air also results in greater quantities of injurious substances such as carbon monoxide and nitrogen oxides.